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Monday 18th November. For some time I have been mulling about using a Skywatcher focuser motor to drive the PST etalon adjustment remotely. While I'm there I really ought to have another motor on the helical focuser. The adjustments required are often too small to see. Those who have never tried a motor driven focuser would be surprised how a tiny degree of rotation can make a huge difference in appearance of the camera's image on the monitor.
To that end I have measured the pull required at a tangent from the rim of my PST etalon. I arranged a thin cord in the form of a Prusik loop wrapped around the PST etalon band. This showed that I needed 2.8lbs of weight to make the ring rotate smoothly without stopping. A small excess of force will be useful for reliability.
The Skywatcher 12V gearbox motor easily raises most loads on the FT focuser. So I added 2.8 lbs to the Lacerta prism and the motor could still raise that weight on 12V. Though there is the matter of the ratio between the pinion and its rack. I'm talking about the focuser's direct drive knob here. Not the 10:1, slow motion knob. One rotation of either of the direct drive [black] knobs causes a 20mm movement of the drawtube and its rack.
To calculate the ratio you measure the movement of the rack caused by one full rotation of the driving pinion. One rotation is the active circumference of the pinion. A small pinion will provide a different ratio from a large pinion. The number of teeth on the pinion or the rack don't matter for this example.
Where a belt drive is used, from the motor to the etalon band, the tiny pulley on the motor provides a lower gear ratio than if both were the same size. The actual number of teeth is again irrelevant because the etalon ring is structured rather than toothed. The drive relies on friction between the toothed belt and the etalon ring.
Now I ought to wrap a thin cord around the Skywatcher pinion to measure its ability to pull.Yet again I used a cord Prusik loop. By locking the cord to the toothed, timing belt I could hang weights from the belt while the motor was used to raise them. There was no hesitation in lifting a 1.5kg weight. [3.3lbs] There is a gear reduction ratio between the small pulley on the motor relative to the diameter of the etalon band. The ratio can be multiplied by the motor's ability to lift weights.The PST etalon band is 60mm in diameter. The present, timing pulley on the motor is 9mm Ø. So there is a minimum 10:1 increase in torque from the motor pulley to the etalon band.
Without doing any maths it is obvious the the motor can easily turn the etalon band. This is provided there is enough friction between the toothed belt and the etalon band. The band is rather slippery [slick] so may need some tension on the belt to achieve a reliable drive. Alternatively a "grippier" surface could be added to the band or the metal band beneath it. A rubber band. Or even better, an inverted timing belt, stretched tightly over the band with its teeth outwards.
The problem is calculating the exact dimensions of a suitable timing belt to grip the etalon band inside out. There is a risk that the belt's teeth will splæay to cause pitch problems. I'll use the same GT2 [2mm pitch x 8mm wide] pulleys and belts. Just as I did on my big, FT focuser. I have duly ordered two Skywatcher motors. The other will be used for the helical focuser.
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